This invention relates to oriented tissue-equivalent and biopolymer tubes and in particular bioartificial arteries made of Type I collagen formed into tubes having collagen fibrils circumferentially oriented by a magnetic field. The present invention additionally relates to tissue regeneration structures, and in particular tissue-equivalent and biopolymer rods for nerve regeneration.
The demand for artificial blood vessels, and in particular, artificial arteries is great. This is due to the fact that over 600,000 surgical procedures involving large and small blood vessels are conducted annually.
Arteries are complex structures, as the cellular and extracellular components of the artery wall are not uniformly distributed. Rather, these cellular and extracellular components are organized into discrete layers. These discrete layers have a trilaminant structure of an intima, a media and an adventitia.
The mechanical properties of the artery wall are largely due to components of the vessel media layer. Elastin is the most extensible component of the vessel media, whereas collagen is relatively stiff. The presence of smooth muscle cells is important for two reasons. First, the smooth muscle cells synthesize the elastin and collagen present in the vessel media and second, they are capable of contraction and relaxation in response to various external stimuli including vasoactive substances in the blood and sympathetic and parasympathetic nerve impulses. These smooth muscle cells are thus an important source of structural protein that contribute directly to the strength and elasticity of the vessel wall and represent a component of variable elasticity participating in the regulation of vessel tone.
The elastin and collagen components confer a non-linear stress-strain relationship, in which the elastic modulus of the vessel wall increases (the vessel becomes stiffer) as the vessel is distended. The importance of organization to mechanical function in the arterial wall is clearly seen when arteries are compared to veins of similar size. In arteries, the circumferentially or (helically) oriented smooth muscle cells and associated elastin and collagen of the media, provide the mechanical strength necessary to withstand the higher pressures that exist in the arterial circulation. In contrast, the media layer of a vein of similar size is reduced in thickness, contains fewer smooth muscle cells, less elastin, and lastly little circumferential orientation of smooth muscle collagen and elastin seen as in the arteries.
Several attempts have been made at creating a completely bioartificial artery. Weinberg and Bell, xe2x80x9cA Blood Vessel Model Construction from Collagen and Cultured Vascular Cellsxe2x80x9d in Science, Vol. 231 (1986), pp. 397-400, reported production of an artery involving smooth muscle cells and fibroblasts in layers of reconstituted Type I collagen that approximate the laminate structure, but not the circumferential orientation of natural arteries. These vessel analogs lacked the mechanical strength necessary to withstand the stress associated with pulsatile blood flow in vivo. To increase the strength, proponents of the Weinberg and Bell approach were forced to reinforce these arteries with synthetic polymer sheaths, such as Dacron(copyright). These bioartificial arteries exhibit a severe drawback in that biocompatability problems can ultimately lead to graft failure as reported by Langer and Vacanti, xe2x80x9cTissue Engineeringxe2x80x9d in Science, Vol. 260 (1993), pp. 920-926. Kanada et al., xe2x80x9cMechanical Stress Induced Cellular Orientation and Phenotypic Modulation of 3-D Cultured Smooth Muscle Cellsxe2x80x9d in ASAIO Journal, (1993), pp. M686-M690, reported a biohybrid vessel which includes polyurethane in order to obtain the appropriate mechanical properties.
The orientation of fibrils and cells in collagen has been studied. For example, Torbet and Ronziere, xe2x80x9cMagnetic Alignment of Collagen During Self-Assemblyxe2x80x9d in Biochem. J., Vol. 219 (1984), pp. 1057-1059, reported collagen fibril orientation in the presence of a magnetic field. Additionally, Guido and Tranquillo, xe2x80x9cA Methodology for the Systematic and Quantitative Study of Cell Contact Guidance in Oriented Collagen Gelsxe2x80x9d in Journal of Cell Science, Vol. 105 (1993), pp. 317-331, reported that cells orient in the same direction as the oriented collagen fibrils, under the phenomen known as xe2x80x9ccell contact guidancexe2x80x9d.
Despite the study of fibril and cell orientation, there remains the need to provide a tissue-equivalent tube that has mechanical properties that enable it-to better withstand distention, typically associated with pulsatile blood flow in arteries, without reinforcing synthetic materials.
Additionally, regeneration of severed peripheral nerves commonly results in permanent damage and disability because mature neurons do not normally replicate. Once severed, the severed ends of the neurons usually grow in a misdirected manner, such that they do not reestablish the original connection. Numerous attempts have been made to reestablish the connections, including the placement of a tube filled with a matrix for bridging the nerve endings. For Example, Rosen, xe2x80x9cArtificial Nerve Graft Using Collagen as an Extracelluar Matrix for Nerve Repair Compared with Suture Autograft in a Rat Modelxe2x80x9d, in Annals of Plastic Surgery, 25:375-387 (1990) reported slightly inferior nerve regeneration using tubes filled with collagen gel as compared to nerve autografts, whereas Valentini, et al., xe2x80x9cCollagen and Laminin Containing Gels Impede Peripheral Nerve Regeneration through Semipermeable Nerve Guidance Channelsxe2x80x9d, in Experimental Neurology, 98:350-356 (1987) reported greatly inferior nerve regeneration as compared to saline-filled tubes.
The present invention comprises a tissue-equivalent tube that has a cylindrical body made of a reconstituted collagen network with smooth muscle cells interspersed therein. The collagen network is comprised of fibrils that orient circumferentially within the tubular body. The cells are induced by the oriented fibrils to also orient circumferentially. The circumferential orientation of fibrils and cells is characteristic of a native artery and results in improved mechanical properties.
The tissue-equivalent tube is formed by a tubular mold having a central rod and a concentric outer member, aligned to provide a cavity between the central rod and the outer member. The mold includes a longitudinal axis, extending the length of the mold. The cavity is then filled with a solution of collagen and mammalian tissue cells. In one embodiment, the filled tubular mold is then placed into a high strength magnetic field such that the longitudinal axis of the tubular mold is substantially parallel to the direction of the magnetic field. The magnetic field orients the forming collagen fibrils normal to the direction of the magnetic field, such that the collagen fibrils are oriented circumferentially with respect to the tube.
The invention additionally comprises tissue-equivalent and biopolymer rods, along with methods for their manufacture and use. In particular, these tissue-equivalent and biopolymer rods include axially (longitudinally) aligned collagen fibrils that provide a guidance field to exploit the contact response in growth cones at the tips of severed nerve endings (neurite tips) to effectively direct nerve cell growth across the gap between severed nerve endings, regenerating mature nerves.
The tissue-equivalent rods are formed by using a cylindrical (rod-shaped) mold with a cavity therein, including a longitudinal axis extending the length of, the mold and a transverse axis extending the width of the mold. The cavity is then filled with a solution of collagen and mammalian tissue cells, preferably Schwann Cells. In one embodiment, the filled cylindrical mold is then placed into a high strength magnetic field such that the longitudinal axis of the tubular mold is substantially perpendicular to the direction of the magnetic field. The magnetic field aligns (orients) the forming collagen fibrils normal (perpendicular) to the direction of the magnetic field, such that the collagen fibrils are aligned (oriented) substantially parallel to the direction of the longitudinal axis.
The biopolymer rods are formed by using a cylindrical mold with a cavity therein, including a longitudinal axis extending the length of the mold and a transverse axis extending the width of the mold. The cavity is then filled with a solution of collagen and polymer beads, that include encapsulated neurotrophic factors. In one embodiment, the filled cylindrical mold is then placed into a high strength magnetic field such that the longitudinal axis of the cylindrical mold is substantially perpendicular to the direction of the magnetic field. The magnetic field aligns (orients) the forming collagen fibrils normal (perpendicular) to the direction of the magnetic field, such that the collagen fibrils are aligned (oriented) substantially parallel to the direction of the longitudinal axis.